Impedance cardiography has been and continues to become an increasingly important mechanism for determining patient condition both during and following medical procedures.
Impedance cardiography can be considered to fall within the more general category of impedance plethysmography, which refers to the measurement of volume (and thereby flow) changes in the body, as derived from observing changes in electrical impedance. Impedance cardiography is generally known as a noninvasive bioimpedance method for measuring cardiac output. Existing cardiac output measurements are based on the principal that blood is a conductor of electricity and that the electrical impedance of the thorax will change during a cardiac cycle. This change in impedance is caused by the thoracic aortic blood flow which is directly related to the amount of blood ejected from the heart.
U.S. Pat. No. 3,340,867, now Re. 30,101, reissued September 1979 to Kubicek, et al., discloses a method for determining cardiac output by measuring the patient's heart stroke volume. There, an impedance plethysmograph employs two sets of electrodes placed on the neck and chests of patients, to provide an impedance difference signal from the two center electrodes. A constant, low-amplitude, high-frequency alternating current is applied to the outermost pair of electrodes while the innermost pair of electrodes senses the voltage levels above and below the patient's heart. Kubicek's method entails first determining the heart stroke volume from these impedance signals, based on the observation that resistance to a current passed through the chest varies with thoracic aortic blood flow, and from this determination of stroke volume, then estimating the cardiac output.
U.S. Pat. No. 4,450,527, issued to Sramek on May 22, 1984, generally discloses a similar apparatus, model and equation for relating impedance and stroke volume to determine cardiac output. U.S. Pat. No. 5,309,917, issued May 10, 1994, U.S. Pat. No. 5,423,326 issued Jun. 13, 1995, and U.S. Pat. No. 5,443,073 issued Aug. 22, 1995, all of which were issued to Wang, et al., each generally disclose variations of the Kubricek and Sramek methods.
Yet another model and method of impedance cardiography regarding the placement and spacing of electrodes has been proposed by Bernstein. According to Bernstein, stroke volume (SV) is related to the change in impedance (Z) as shown in Equation 1:
(1) ##EQU1## SV = Stroke Volume .delta. = correction factor for patient weight H = Patient height (cm) T.sub.LVE = left ventricular ejection time (sec) (dZ/dt).sub.max = maximum value of the first derivative of Z, where Z is the change in impedance caused by thoracic aortic blood flow Z.sub.0 = mean baseline impedance of the thorax (ohm)
While each these methods can be helpful in determining cardiac output, the various types of non-invasive devices disclosed such as the outer skin electrodes of Kubicek and Sramek, often prove inefficient, for example when dealing with many surgical procedures or with skin abrasion patients. As one can imagine, these devices require a number of exposed connective wires and corresponding electrodes that may interfere with other surgical procedures. Furthermore, because the inner surface electrodes may receive impedance signals from various other regions within the patient due to the distance in placement of the electrodes from the thoracic aorta region, accuracy concerns have been raised. Additionally, incorrect electrode placement can result due to the changes in the patient's physiology of the thorax with respect to the placement of the electrodes on the stemum, as well as due to the size of the patient. Finally, as recognized in Equation 1, a correct factor for patient weight, .delta., must be utilized in calculating cardiac output, and often if the weight cannot be accurately determined, the weight estimation can be another source of inaccuracy.
Several of the problems with prior art non-invasive devices have been addressed by more recent developments; however, these new developments still fall short in many critical areas. For example, U.S. Pat. No. 4,836,214, issued to Sramek on Jun. 6, 1989, generally relates to an esophageal probe comprised of an array of electrical bioimpedance ring electrodes provided on a hollow, flexible tube that is insertable into the esophagus of a patient and positioned proximate the descending thoracic aorta. The Sramek device, however, like the other non-invasive prior art probes, still permits movement of the probe within the esophagus. As a result of this motion artifact, inaccuracies are possible. This problem may be further attenuated by the use of the disclosed ring electrodes in that such electrodes often tend to float within the esophagus.
U.S. Pat. No. 5,357,954, issued to Shigezawa et al. on Oct. 25, 1994, generally relates to an esophageal blood oxygen saturation probe with temperature and sound sensing devices for invasively monitoring a patient. The patent purports to suggest discloses that the internal walls of the esophagus will tend to collapse onto the outer surface of the probe's chassis and sound sensor, such that the probe's sensors will not move appreciably with respect to the esophagus. The ability of the esophagus to prevent undesirable movement of the probe as so disclosed, particularly given the size of the probe, is questioned. Nevertheless, because the probe is not substantially fixed relative a to the esophagus, there still exits an opportunity for undesirable movement which, as will be appreciated by those skilled in the art, can lead to inefficient and less accurate results.
Motion limiting devices such as those disclosed in prior oximetry work of the present assignee are known; however, heretofore teachings have not been used in impedance cardiography applications. In this regard, the subject matter of application Ser. No. 60/045,006, application Ser. No. (U.S. Pat. No. 5,715,816), application Ser. No. 08/412,287 (U.S. Pat. No. 5,743,261) and U.S. Pat. No. 5,417,207 are incorporated herein by reference.
There exists a long felt need, as one skilled in the art will appreciate, for an esophageal probe utilized in impedance cardiography that greatly reduces the movement of the catheter within the esophagus, with little or no motion artifacts. Furthermore, there exists a long felt need for improved electrodes that positively address the limitations of presently known electrodes.